The disclosure concerns a method for damping an oscillation of a roller in a printing system, the roller being driven via a drive, in which a printing substrate web is directed across the roller. The drive drives the roller with a predetermined (in particular constant) nominal moment of inertia. The roller and the printing substrate web form a system that is capable of vibrating.
In the printing system, the printing substrate web is directed across a plurality of rollers, wherein the printing substrate web and the rollers together may form a system capable of vibrating in which a mass of the vibrating system is formed in particular by rollers with a high moment of inertia, and an elasticity is formed due to the printing substrate web.
In particular given what are known as cross-turners that are used in order to turn the printing substrate web between two printers such that the printing substrate web may be printed on both sides, problems often occur due to oscillations since—given such oscillations—the natural frequency of the vibration-capable system may coincide with an excitation frequency, such that it leads to a resonance and the oscillation correspondingly reinforces itself. This leads to high oscillations in a tension within the printing substrate web which may propagate into the image-generating transfer-printing regions of the printing units, and thus may lead to color registration errors. Moreover, due to too high a tension a tear of the printing substrate web may arise, in particular upon printing of perforated webs. Such printing systems also often have monitors for the tension of the printing substrate web that may be triggered by such resource oscillations, which may lead to the shutdown of the printing system. Moreover, slackening may occur which in turn may lead to a synchronization loss upon printing to the front side and the back side.
Cross-turners are particularly susceptible to such resonance oscillations since a cooling roller that has a very large moment of inertia may be built into them. Depending on the velocity with which the printing substrate web is transported through the printing system, an excitation with the different frequencies occurs due to eccentricities of rollers across which the printing substrate web is directed. If this excitation frequency coincides with the natural frequency of the vibration-capable system that is formed by cooling rollers and the printing substrate web, a corresponding resonance occurs.
The natural frequency lies in the range between 4 and 18 Hz, depending on the cross section and material of the printing substrate web. Depending on the velocity, the excitation frequency may be between 0 and 40 Hz, wherein different excitation frequencies occur due to the different diameters of the different installed rollers and the respective eccentricity of these rollers at the same velocity, such that the occurrence of a resonance case is very probable.
A first known possibility to avoid errors due to oscillations is to avoid the printing system being operated in the dangerous resonance ranges. For this, upon varying the velocity with which the printing substrate web is transported these variations are implemented as quickly as possible in order to pass through the resonance range quickly.
However, in this method it is disadvantageous that it is nearly impossible to reliably circumvent the resonance ranges, due to the many different rollers and the different excitation frequencies that they generate and the different natural frequencies depending on the printing substrate web that is used.
An additional known method to avoid problems with oscillations is to move the resonance ranges so that they lie outside of the typical operating states of the printing system. For example, from the document US 2011/0315031 A1 corresponding to U.S. Pat. No. 8,448,572 it is known that, for this purpose, deflection rollers are displaced in order to thus alter the spring constant of the elastic printing substrate web, and thus to change the natural frequency of the vibrating system. However, in this method it is disadvantageous that a complicated adaptation is necessary for every printing substrate web that is used.
An additional known method to reduce resonance problems is that the excitation amplitudes are minimized. One possibility for this is to attempt to minimize the eccentricities of the rollers across which the printing substrate web is directed, such that only a minimum excitation takes place. However, this is linked with very high costs.
An additional possibility to avoid resonance problems is to damp the occurring oscillations. Such a damping may take place mechanically and/or electrically via a controller.
Given a mechanical damping, in particular mechanical viscous dampers are used in which a closed housing is borne on the shaft to be damped so as to form a seal, wherein this sealed housing is filled with silicone oil. A disc is also arranged on the shaft, which disc rotates with the shaft and is arranged within the housing chamber filled with silicone oil. If an oscillation occurs, a moment proportional to the change of velocity results due to the viscous friction, which moment is directed counter to the oscillation and thus effects a damping at every point in time.
The disadvantage of such a mechanical viscous damper is that these are expensive, have a high weight, and take up a great deal of scarce structural space. Moreover, such mechanical viscous dampers lead to problems upon acceleration and braking of the driven roller since the viscous dampers must be accelerated or braked as well, such that an unnecessary expenditure of force and energy arises and the acceleration process or braking process is delayed. Moreover, heat is created due to the friction between the disc and the silicone oil, which heat may lead to a significant heating of the viscomechanical damper. Such viscomechanical dampers may also be modified to different application cases only at great expense.
Given electrical damping, the oscillation is determined and the control of the drive unit of the roller is adapted accordingly such that the oscillation is damped.
For this, from the document EP 1 837 178 A2 a method is known for compensation of a vibration in which a frequency spectrum of the vibration is determined and is divided up into frequency portions, wherein multiple counter-moments are determined via the division into the frequency portions, via which counter-moments the vibrations are compensated. In this method it is disadvantageous that the recording of system parameters is necessary for this, and in particular all excitation frequencies and amplitudes of the printing system must be determined in a complicated manner for this. In particular, expensive sensors are necessary for this.
From the document DE 101 07 135 A1, a method for vibration damping in a printing machine is known in which multiple rollers roll on one another. An active damping is hereby provided in which occurring position deviations of at least one roller are detected and respective countering damping forces are provided at at least one roller.
The document DE 10 2007 006 683 A1 describes a method for active vibration damping given counter-rotating rollers. Used for this are: at least one sensor for detection of the vibration at at least one roller; a regulator to process the vibration data detected by the at least one sensor and to emit at least one control signal based on these vibration data; as well as at least one actuator that charges at least one of the rollers with one of the forces counteracting the vibration on the basis of the at least one control signal.